As a kind of external-combustion engine, a Carnot engine is an ideal engine, and thus all actual engines cannot surpass the thermal efficiency of the Carnot engine. As compared with an internal-combustion engine, the external-combustion engine has a high thermal efficiency, low vibration and low noise, and is also an environment-friendly engine that emits fewer pollutants while fuel burns up. Further, only an external heat source and a medium (e.g., a heat sink) needed for cooling, i.e., only the media having temperature difference are enough to operate the external-combustion engine, and the external-combustion engine can be operated by all energy sources such as solar heat, etc.
Among the external-combustion engines, a commercial engine includes a Stirling engine and a turbine engine based on a Brayton cycle. The Stirling engine is a kind of external-combustion engine in which a space formed by a cylinder and a piston and filled with hydrogen, helium, or the like operating gas is sealed up, and the piston is moved up and down as the operating gas is heated and cooled through heat exchange by the exterior, thereby getting mechanical energy. The Stirling engine has high thermal efficiency similar to that of the Carnot engine.
However, the whole size and structure of the above Stirling engine are respectively so large and complicated that high manufacturing costs and a high level of technology are disadvantageously required. Further, the Stirling engine is disadvantageously applicable to only a restricted field due to its difficult maintenance.
Also, most engines change a rectilinear motion due to thermal expansion of gas inside the cylinder into a rotary motion using a crank, in which much power loss is caused by friction. Accordingly, a linear thermal engine for driving a leaner generator configured with a free piston is being developed, but is still in a developmental stage because it has low efficiency and many problems with controlling noise and vibration.